Electric charge is a polarity of matter that

governs how matter behaves in electric or

magnetic field.

The SI – unit of charge is coulomb (c)

Coulomb is the quantity of electric charge

which passes through a given point in a

circuit in one second when steady current of

1 Ampere is flowing.

Coulomb is the SI unit of electric charges

Note that, the electron has the smallest

amount of charge, e = - 1.6 ×10

-19

C and that

of proton

p = + 1.6 ×10

-19

Coulomb is a much larger quantity of charge

than that is normally produced by rubbing

hence we use micro – coulomb (μC) in

measuring charge.

1μC= 1 ×10

-6

To get the amount “q” of charges we use the

formula q=ne

Whereby n= number of particles

FUNDAMENTAL LAW OF STATIC

ELECTRICITY

This law is also known as the first law of

electrostatic, states that:

“Like charges repel, unlike charges attract”

Example 01

If a comb and a piece of glass rod are both

rubbed with a piece of woolen fabric and

brought together.

(a) What material becomes positively

and which becomes negatively

charged?

Answer

The comb becomes positively

charged while the rod becomes

negatively charged

(b) What will be observed if the comb

and a glass rod are brought nearby?

Answer

The comb and the glass rod attract

each other.

exist between them

Answer

The type of electrostatic force

existing is attractive force.

Example 02

Two rubber balloons are rubbed with hair.

the balloons and the hair be attractive

or repulsive? Explain

Answer

The electric force between one of the

balloons and the hair will be attractive

because they acquire different charges.

Unlike charges attract.

(d) Will the electric force between the two

balloons be attractive or repulsive?

Explain.

Answer

The electric force between the two

balloons will be repulsive, because

they acquire the same type of charges.

Like charges repel.

TEST FOR POLARITY

The polarity is the only sure way for

determining the polarity of the charge

because it occurs only between two bodies

of the same charges.

Attraction is not a sure test because.

(i) Attraction can occur between two

bodies of opposite charges.

(ii) Attraction can occur between

uncharged body and a charged body.

Therefore, repulsion is the only sure test for

charges.

CHARGING (electrification).

Charging a body is the process of giving an

electric charge to a body by adding or

removing electrons.

Methods of charging a body

A body can be charged by three methods,

namely

(i) Charging by friction

(ii) Charging by contact

(iii)Charging by induction

Charging by friction (rubbing)

Charging by friction is the process of

rubbing two objects together to create a net

charge on both objects.

During rubbing, the electrons flow from one

object to another, this creates deficit of

electrons in one object and excess electrons

in one object

The object that loses electrons becomes

positively charged while which gains

electrons becomes negatively charged. The

body which has equal positive and negative

charges is called a neutral body.

Example 01

Inflammable liquids are not transported

through plastic pipes. Give a reason for this.

Answer

When a liquid flows through a pipe, its

molecules become charged due to rubbing

on the inner surface of the pipe. If the liquid

is flammable, it can cause sparks and

explode.

Note that: it is not advisable to store highly

flammable liquid in plastic can to avoid

explosion due to rubbing (friction) of liquid

molecules against the walls of the can which

cause accumulation of static charges.

Example 02

Some materials, mainly plastics acquire

charges when rubbed. This enables them

attract smaller particles e.g. small pieces of

paper and dust, Give three daily applications

of the phenomenon in life.

Answer

(i) Rubbing a plastic ruler on dry hair make

it attract some small pieces of paper or

dust

(ii) Household mirrors and windows attract

dust and other small particles when

wiped with a dry duster.

(iii)When a nylon dress is removed from the

body, a cracking sound is heard.

Example 03

Fuel tankers have a lose chain hanging

under them to touch the ground as they

move. Explain.

Answer

The loose chain is used to earth the charges

produced by friction between the moving

parts of the vehicle.

Charging by contact (conduction)

Charging by contact is a method of charging

an object by touching it with another object

that is already charged.

When a charged body touches a neutral

body, they share equal amount of the same

charges after separation.

Charging by electrostatic induction

Charging by induction is a method of

charging an object without touching it with a

charged body.

Uncharged body acquires opposite charge to

that of a charged body by bringing a charged

body nearby a neutral body.

Charging a body positively by induction

If you want to charge a body positively,

bring a negatively charged body nearby.

Positive charges are induced on the side near

the charged body and negative charges are

repelled to the other side.

With the charged body still in position, the

other side of the conductor is earthed by

connecting it with an earth wire to the

ground. Electrons flow to the ground

through the earth wire.

With the charged body still in position, the

earthing is removed and then the charged

body is also removed. Positive charges

distribute themselves all over the conductor

and the conductor is left with a net positive

charge.

Charging a body negatively by induction

If you want to charge a body negatively,

bring appositively charged body nearby.

Negative charges are induced on the side

near the charged body and positive charges

are repelled to the other side

With the charged body still in position, the

other side of the conductor is earthed by

connecting it with an earth wire to the

ground. Electrons flow from the ground and

neutralize the positive charges on the

conductor.

With the charged body still in position, the

earthing is removed and then the charged

body is also removed. Negative charges

distribute themselves all over the conductor

and the conductor is left with a net negative

charge.

Points to note: Earthing a conductor may

also be done by touching it with the fingers

since human bodies are good conductors of

electricity.

SPECIAL CASES

Case I: charging two bodies at the same

time with opposite charges

If you want to charge two bodies A and B at

the same time with opposite charges, bring a

negatively charged body near A as shown

below.

Positive charges are induced on conductor A

and negative charges repelled to conductor

With the charged body still in position, the

conductors are separated. On removing the

charged body, the charges distribute all over

the conductors. Conductor A acquires a net

positive charge and conductor B acquires a

net negative charge.

Case II: charging two bodies at the same

time with the same charges

If you want to charge two bodies A and B at

the same time with the same charges, bring a

negatively charged body near A as shown

below.

The body A acquires positive charge while

body B acquires negative charge.

With the charged body still in position,

conductor B is earthed by connecting it with

an earth wire to the ground. Electrons flow

to the ground.

With the charged body still in position, the

earthing is removed and then the charged

body is also removed. Positive charges

distribute themselves all over the two

conductors and the conductors are left with a

net positive charge.